This application relates to power supplies for motors, and more particularly to a power supply for a motor of a blower used in a healthcare environment which filters Electromagnetic Interference EMI emissions and reduces ground current leakage.
Recently hospitals have been replacing standard mattresses and patient support apparatus with air inflatable, low air loss, and/or fluidized mattresses which require the use of an air supply incorporating a blower to maintain proper pressure levels. These blowers are typically operated using motors driven by power conversion circuits which generate unwanted signals. These unwanted signals can affect, or interfere with, other electronic systems and with the control system of the blower itself. These effects of the unwanted signals are commonly called electromagnetic interference (EMI) which is also known as radio frequency interference (RFI). EMI emissions can be transmitted by radiation through space or by conduction along supply lines.
Many electrical systems are susceptible to EMI. In the hospital environment EMI emissions can adversely affect critical equipment causing catastrophic results. Some evidence indicates that EMI emissions can cause electric powered wheelchairs to exhibit erratic behavior, cause incorrect readings from adult heart monitors leading to improperly administered defibrillation, cause failure of alarms in apnea monitors and infant heart monitors, and induce ventilators to change their breath rates.
In the United States EMI emission standards are set by the Federal Communication Commission (FCC). The FCC has promulgated standards of permissible radiated and conducted radio frequency emissions for common commercial products as well as products intended for commercial or industrial use in The FCC Rules and Regulations, Part 15, Subpart B. The radiated and conducted test procedures are defined in ANSI Standard C63.4. The Food and Drug Administration (FDA) through its Center for Devices and Radiological Health (CDRH) has also considered promulgating standards for devices to be used in hospital environments and has worked to heighten EMI awareness among medical device manufactures. The FDA also requires immunity testing for medical equipment. The European Economic Community has adopted standards governing not only EMI emissions, but also EMI susceptibility, of electronic devices. Compliance with EEC""s conducted EMI emissions is established by passing Underwriters Laboratory (UL) EN55011 Class A emissions test. In order to comply with all of the standards promulgated by the FCC, electronic devices incorporate electromagnetic compatibility (EMC) systems.
EMI is created by rapid variations of voltages or currents within a system. The magnitude of the EMI depends on the value of the current, the length of the conductors, the rate of change of voltage and current, and the physical position of the conductors relative to each other and any earth planes. In blowers, the rapid variations can be caused by, among other things, the arcing of motor commutators (when motors with commutators are used) or the rapid switching of the motor controller components. One method of reducing conducted EMI emissions is to introduce an EMI line filter between the main supply lines and the load. These filters use a circuit combining inductors and capacitors (LC circuit) to reduce the EMI emissions of the system. The capacitors of the LC filter circuits are typically connected between the supply lines and earth ground. While the LC filter circuit effectively reduces EMI emissions, it also generates a ground leakage current across the capacitors.
Ground leakage current in medical devices is also regulated by the FDA. Medical patients often have direct and intentional contact with medical equipment which could result in the ground leakage current flowing through the patient to ground. Thus standard LC filters are not adapted for use in medical equipment because of the serious hazard they pose to patient safety. Stringent EMC specifications are imposed upon equipment used in the medical industry, with even higher specifications being imposed on equipment classified in the xe2x80x9cpatient contactxe2x80x9d category.
Special EMI line filters have been developed for use with medical equipment which reduce or eliminate ground leakage current. Among the medical application EMI filters are the LM series of EMI filters available from Filter Concepts, Inc. of Santa Ana, Calif., USA and the EHQ series of EMI Filters available from Corcom, GmbH in Martinsried, Germany a wholly owned subsidiary of CII Technologies Corcom Division. Both of these filters"" circuitry include a plurality of inductors, a plurality of capacitors, and one or more resistors connected between the line and the load.
According to the present invention an EMI filter for use with a motor driven blower connectable to an inflatable portion of a patient support powered by two supply lines includes a blower ground isolated from chassis ground and earth ground, a first capacitor extending between and directly connected to the blower ground and one of the two supply lines, and a second capacitor extending between and directly connected to the blower ground and the other of the two supply lines. The EMI filter may also include a third capacitor extending between and directly connected to both of the two supply lines.
According to the present invention, a portable air supply unit powered by a supply having two current carrying lines and connectable to an inflatable patient supporting apparatus includes a chassis having an electrical potential defining chassis ground, an electrically powered blower received in the chassis and connected to each of the two supply lines, insulation and shielding isolating the blower from the chassis to define an electrical potential of the blower defining blower ground, a first capacitor extending between and connected to one of the two supply lines and blower ground, and a second capacitor extending between and connected to the other of the two supply lines and blower ground. The portable air supply might also include a third capacitor extending between the two supply lines.
A method of reducing conductive EMI emissions and ground leakage current of a blower for medical equipment powered by a two line current supply includes the steps of connecting a blower for operation to the two line power supply, providing a chassis, isolating the blower from the chassis to define a blower ground isolated from chassis ground, capacitively coupling each of the supply lines to blower ground.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.